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Anomalous Z Cam stars: a response to mass-transfer outbursts

Recent observations of two unusual Z Cam systems, V513 Cas and IW And, by Szkody et al. (2013) have shown light-curves that seem to contradict the disc instability model for dwarf novae: outbursts are appearing during standstills of the system when, according to the model, the disc is supposed to be in a hot quasi-equilibrium state. We investigate what additional physical processes should be included in the model to reconcile it with observations of such anomalous Z Cam systems. We use our code for modeling thermal-viscous outbursts of the accretion discs, and determine what kind of mass-transfer variations reproduce the observed light curves. We find that outbursts of mass transfer (duration a few days, with a short rise time and an exponential decay) from the stellar companion will account for the observed properties of V513 Cas and IW And, provided they are followed by a short but significant mass-transfer dip. The total mass involved in outbursts is of the order of 1023g. Conclusions: We study the possible origins of such mass transfer outbursts, and show that they most probably result from a giant flare near the secondary star surface, possibly due to absence of star-spots in the L1 region.